The fact that language is a conventional system, used by all speakers in a community to convey the same meanings, underlies our success as communicators and provides the basis for acquiring language. The proposed research will investigate the origins of key aspects of conventionality during the period in which the first words are acquired. First, these studies will provide some of the first experimental work investigating when infants recognize conventionality in language. Second, this research will examine the generality of infants' assumptions by comparing word use to other types of human action. Third, by testing infants' understanding of newly introduced words, the project will assess the degree to which infants' understanding of conventionality contributes to word learning. Finally, because the proposed experiments distinguish between different aspects of conventionality, the project will bring greater precision to current thinking on this topic. The proposed studies are intended to provide initial vantage points into three aspects of conventional knowledge: (1) Knowing that all speakers (within a community) use the same form-meaning pairings; (2) Knowing that these pairings form a contrastive system; and (3) Knowing that conventional systems can, in principle, vary across speakers. In addition to providing a foundation for further investigations, these studies will shed new light on general processes in early language learning. As such, the findings may contribute to scientific and professional knowledge about cases in which development goes awry, for example, in cases of language delay, or more global communicative deficits such as those present in autism. In addition, because these studies concern the development of knowledge about the systems shared by language communities, the findings may ultimately contribute to our understanding of multi-lingual language development. [unreadable] [unreadable] [unreadable]